a) Field of the Invention
This inventions relates to acrylamide polymers and aqueous solutions thereof, which are useful in papermaking industry, the field of effluent treatment, the field of civil engineering and construction, and the like. Specifically, the present invention is concerned with acrylamide polymers having a controlled branched and crosslinked structure. The acrylamide polymers according to the present invention are particularly useful in the field of papermaking industry as paper strength agents excellent in paper reinforcing effects.
b) Description of the Related Art
Acrylamide polymers have been used for various applications to date. Specifically, paper strength agents, drainage agents, retention aids, coagulants, secondary oil recovering agents, dispersants, etc. In these applications, various acrylamide polymers whose weight average molecular weights range from about 100,000 to somewhat greater than 20,000,000 are chosen depending on the application purposes, and those having a linear molecular structure have been used primarily.
In recent years, it has been attempted to improve the performance of acrylamide polymers in the field of papermaking industry, such as paper strength agents, by allowing them to have a partially-crosslinked structure and a partially-branched structure so that their molecular weights can be increased while suppressing a viscosity increase in their solutions.
There is, for example, a technique in which a vinyl monomer having two or more double bonds in a molecule is used as a crosslinking agent. Specific known examples of the vinyl monomer include divinyl monomers such as methylenebisacrylamide, methylenebismethacrylamide, ethylene glycol diacrylate and ethylene glycol dimethacrylate; and trifunctional monomers such as 1,3,5-triacryloylhexahydro-S-triazine and triallyl isocyanurate. The molecular weight increasing technique, which relies upon these vinyl monomers alone, are however still considered to be insufficient for paper strength agents, because it results in a branched and crosslinked structure substantially lacking uniformity and moreover, increased gelling effects or the like are observed when polyfunctional monomers are used.
It has also been attempted to produce an acrylamide polymer having a branched and crosslinked structure by using a specific compound in combination with such a crosslinking agent. Known combinations include, for example, the combinations of crosslinking agents and primary amines or OH-containing monomers, the combinations of crosslinking agents and specific hydrophobic monomers, and the combinations of crosslinking agents and itaconic acid derivatives. Although it is said that these combinations can all achieve an increase in molecular weight while maintaining the viscosity low but in view of the structures of the resulting polymers, they are not sufficient as paper strength agents and their effects are still insufficient.
To produce acrylamide polymers having a branched and crosslinked structure, attempts have also been made using means other than the above-described branching and crosslinking method which relies upon copolymerization making use of a polyfunctional vinyl monomer. As one example of this approach, a papermaking additive has been proposed, which uses a reaction between a specific N-substituted acrylamide derivative such as N,N-dimethylacrylamide and a persulfate- or peroxide-base catalyst. This method is said to permit an increase in molecular weight while maintaining the viscosity low but, because of occurrence of polymer degradations, the resulting polymer is not sufficient in view of its structure and is not considered to exhibit sufficient effects as a paper strength agent.
The conventional acrylamide polymers produced by these conventional techniques are generally used as aqueous solutions for the above-described applications. Processes have been proposed for the production of an acrylamide polymer which has a molecular weight of about 3,000,000 or so and can be formed into an 15% aqueous solution. No acrylamide polymer solution having a high concentration is however known at all. The concentration is as low as 15% to 21% at most, because a linear acrylamide polymer which may be formed into an aqueous solution having a concentration higher than the above level does not have such a high molecular weight as desired or the solution so formed has a very high viscosity. In addition, any attempt to introduce a branched and crosslinked structure into an acrylamide polymer causes a crosslinking reaction to locally proceed to a considerable extent so that, coupled with the high concentration, water-insoluble gel is formed or the resulting mixture is gelated entirely and the resulting acrylamide polymer cannot be obtained in the form of an aqueous solution. The acrylamide polymer is therefore not usable as a paper strength agent.
An application for patent was filed in Japan on a surface strength agent composed of an aqueous solution of a polyacrylamide polymer, which has a viscosity of 500-5,000 cps and a concentration of 10-30%. The application was published under Japanese Patent Laid-Open No. 279491/1991. Although a molecular weight range of 100,000 to 3,000,000 is referred to in its specification, there is no specific disclosure as to the possibility that the process disclosed therein may achieve a molecular weight increase to a level as high as 500,000 or greater in terms of absolute molecular weight while permitting a concentration of 22% or higher.
Despite a merit available in shipping cost from a higher polymer concentration, no aqueous solution containing a high molecular weight acrylamide polymer at a concentration of 22% or higher was known at all to date for the reasons described above.